Due to global spread of the internet and an increase in “things” connected to the internet, the amount of data traffic has dramatically increased, and improvement of transmission capacity and transmission speed is required. The performance of an information processing system is determined by the processing speed of the computing device and the transmission speed of a mutually coupling medium. Regarding the computing device, research and studies are continuously made on scaling or miniaturization of silicon integrated circuits. Following the scaling of IC devices, scaling of the mutually coupled medium and improvement of operation speed are also required. In the future, further downsizing and high-speed operation will be desired for the entirety of a transmission system.
For downsizing and speedup of systems, mutual coupling in optical transmission using silicon integrated circuits and fiber optic cables is attracting attention. One solution for increasing the number of channels per fiber is a technique of wavelength division multiplexing (WDM). At a receiving end, a WDM signal is demultiplexed into light signals with different wavelengths.
An optical demultiplexer in which a plurality of delay-line Mach-Zehnder (MZ) interferometers are cascaded is known. In this architecture, two outputs of the first delay-line MZ interferometer are connected to an input of the second delay-line MZ interferometer and an input of the third delay-line MZ interferometer, respectively. Each of the second and the third delay-line MZ interferometers has half the path length difference of the first delay-line MZ interferometers. See, for example, Patent Document 1 listed below.
An optical demultiplexer in which a plurality of directional optical couplers are cascaded, is also known. See, for example, patent Document 2 listed below. With this architecture, each directional optical coupler has an input port, an output port, and two optical waveguides extending between the input port and the output port. The relative effective path lengths of the two optical waveguides are regulated such that the direct current component of the optical signal detected from the output of the optical demultiplexer becomes the maximum.
FIG. 1 illustrates a conventional optical demultiplexer for demultiplexing a WDM signal. A received WDM signal is input to the optical demultiplexer in which a plurality of asymmetric MZ interferometers (which may be referred to as “AMZs”) are cascaded in a tree structure. In this example, a WDM signal with four signal components of wavelengths λ1 to λ4 that are multiplexed is input to the optical demultiplexer.
The path length difference of the first stage AMZA is 2ΔL, and the path length difference of the second stage AMZB and AMZC is ΔL. The WDM signal is separated into two transmission components whose spectra are inverted relative to each other, at two output ports of the AMZA. The AMZA is designed such that the upper arm transmits the lights of λ1 and λ3 and the lower arm transmits the lights of λ2 and λ4. The separated transmission spectra are input to the second stage AMZB and AMZC, respectively. In each AMZ of the second stage, the inputted spectrum is again separated into two transmission components whose spectra are inverted relative to each other. The period of the transmission spectrum is inversely proportional to the arm length difference. The period of the transmission spectra separated by the second stage AMZB and AMBC is twice as long as that of the transmission spectra separated by the first stage AMBA. With this structure, signals of four wavelengths are separated at and output from the total of four output ports of the second stage AMZs.
The spectrum of the light transmitted from the input port to each of the output ports of the optical demultiplexer is determined by the product of the transmission spectra of the AMZs existing between the input port and the output port. By finely tuning the effective arm length difference of each AMZ at the order of the wavelength to bring the peak wavelength of the transmission spectrum to an appropriate wavelength for the input WDM signal, a spectrum from which a light component of a target wavelength can be separated is obtained at the output port.